1. Field of the Invention
The present invention relates to suspension systems and, more particularly, to an electronically-controlled shock absorber unit having variable damping and spring forces.
2. Description of the Prior Art
Presently, there are basically two categories of automotive suspension systems. The conventional passive suspension systems use relatively soft automotive suspension springs for absorbing irregularities in road surfaces without transmitting excessive acceleration forces to the vehicle. Being designed for comfortable ride results, these passive suspension systems have thus a problem of excessive vehicle body motion in response to longitudinal and lateral accelerations. For example, excessive high roll angles in turns, poor stability when driving fast, excessive vehicle attitude change during acceleration or braking, bottoming on rough roads and other detrimental effects therefore result. A hard suspension would eliminate these problems but would lead to a deterioration of riding comfort due to increased transmission of vibrations from the road. A conflict therefore exists between the requirements for good ride comfort which are provided by soft suspension systems and acceptable handling and stability which are provided with harder suspensions. In order to resolve this, the suspension characteristics have been made adjustable depending on the driving conditions.
Passive suspension systems are undoubtedly the most simple, inexpensive and reliable means for protecting vehicles from the shock and vibration environment. However, the aforementioned performance limitations of these systems due to their fixed parameter nature are well known. It has been established that suspensions with adjustable parameters that can be varied to suit changing excitations and response characteristics can provide better performance than the above-mentioned passive systems.
Active suspension systems comprising parameters that are automatically readjusted with response and excitation variables have been developed. Although these systems demonstrate superior performance than the best possible passive systems, the active systems are generally more costly, more complex and therefore less reliable than their passive counterparts.
U.S. Pat. No. 4,685,698 issued Aug. 11, 1987 to Klinkner et al discloses a shock absorber having adjustable damping characteristics which comprises two chambers that are connected by a controllable bypass. By way of a controllable throttle or valve provided in the bypass, the damping characteristics of the shock absorber can be adjusted. Acceleration sensors mounted on the vehicle for producing electrical signals which are representative of road surface undulations and irregularities provide sensor signals which are processed by a processing circuitry which produces control signals for proper adjustment of the throttle or valve.
In U.S. Pat. No. 4,645,042 issued Feb. 24, 1987 to Inove et al, a hydraulic damper has adjustable damping characteristics which are produced by the adjustment of a liquid passage connecting a pair of liquid chambers disposed on each side of a piston working in a cylinder. A member is rotatably disposed in the liquid passage for selectively adjusting the effective passage area of the liquid passage. At least three different passage areas are provided. The improvement lies in an actuating device for rotating the adjusting member for providing variations of the effective cross-sectional area of the liquid passage between the two liquid chambers.
U.S. Pat. No. 4,702,490 issued Oct. 27, 1987 to Yamaguchi et al discloses an automotive suspension system having adjusting damping characteristics according to driving conditions. As above, a piston in a cylinder defines a pair of chambers filled with a working fluid. A pressure control valve is adapted for selectively establishing and blocking fluid communication between the two chambers. In response to a relative displacement between a body of a vehicle and a road wheel assembly, means control the control valve for adjusting the fluid pressure in the chambers for adjusting the relative distance between the vehicle body and the road wheel assembly.
U.S. Pat. No. 4,700,971 issued Oct. 20, 1987 to Doi et al discloses an active suspension system that detects an absolute acceleration in a passenger's compartment of a vehicle and a relative displacement between an axle and a body for controlling the damping force of hydro-pneumatic springs, thereby improving steering stability and riding comfort.
All of the above described suspension systems disclose apparatuses for selectively adjusting the damping characteristics of a shock absorber. Adjustability of the spring force as well as of the damping force is not provided.